1. Field of the Invention
The present invention relates to a frame structure of a vehicle that is designed to accommodate a running-power transmission mechanism and constitute at least a part of a vehicle frame.
2. Related Art
A known frame structure as disclosed such as in Japanese Patent Application Laid-open No. Hei-08-2267 is formed by connection members made up of a flywheel housing, an intermediate housing and/or a transmission case that are connected to each other along a longitudinal axis of the vehicle (hereinafter simply referred to a vehicle longitudinal axis).
The frame structure of the above type has an insufficient capability in a point as discussed below.
First of all, the frame structure of the above-cited reference includes a front housing (flywheel housing) that accommodates a flywheel and a sub-speed change unit, a transmission case connected to the front housing and a rear housing connected to the transmission case. Specifically, the front housing has a partition wall formed integral therewith substantially at the center of the front housing with respect to the vehicle longitudinal axis. The partition wall supports a drive shaft and an intermediate shaft of the sub-speed change unit in cooperation with a bearing frame that is connected to a rear side of the front housing.
According to the above arrangement, the assembling of the frame structure involves troublesome works, which include connecting the bearing frame to the rear side of the front housing while supporting the rear ends of the drive shaft and the intermediate shaft on the bearing frame, with the front ends of the drive shaft and the intermediate shaft being supported on the partition wall and the other constitutional members of the sub-speed change unit such as a friction clutch being placed within the front housing.
Another problem caused by the frame structure of the above-cited reference is that the transmission case is hard to be reduced in size since the transmission case, which accommodates a main-speed change unit, is located in the intermediate position between the front housing and the rear housing. Specifically, in the above-cited reference, transmission shafts (a hollow propeller shaft and a main shaft inserted therethrough) are disposed coaxially with a crank shaft of the engine, aiming at the reduction of the size of the front housing and the like. However, this arrangement requires the transmission shaft to be located with sufficient distance from a wall of the transmission case to provide a space for various transmission members such as gears and clutches to be mounted on the transmission shafts. Accordingly, a problem in that the transmission case is hard to be reduced in size cannot be addressed.
Another problem also lies in an arrangement of the vehicle structure of the above-cited reference in a case where a part or all of the inner space of the vehicle structure is utilized as a hydraulic fluid reservoir tank. Specifically, the hydraulic fluid stored in the reservoir tank is utilized for various hydraulic mechanisms mounted in the vehicle. Therefore, a sufficient amount of hydraulic fluid must be stored inside of the frame structure in order to avoid fluid running-out for the hydraulic mechanisms in a case where the frame structure is also used as a reservoir tank.
Specifically, variation in position or posture of the vehicle is directly reflected in the position or posture of the frame structure, since it constitutes a portion of the vehicle frame. That is, when the vehicle tilts due to such as running up or down a slope, the frame structure takes the same tilting position or posture. In such a tilting position or posture, the surface level of stored fluid is changed. Accordingly, fluid running-out for the hydraulic mechanisms is highly likely to occur when the amount of fluid stored in the reservoir tank is small. Therefore, it is necessary to store a sufficient amount of fluid inside of the frame structure in a case where the frame structure is also used as a reservoir tank.
On the other hand, hydraulic fluid stored inside of the frame structure may cause drag resistance in various transmission mechanisms placed inside of the frame structure. That is, a large amount of fluid stored inside of the frame structure may deteriorate transmission efficiency of various transmission mechanisms that are mounted also inside of the frame structure and hence immersed in the fluid.
Thus, there exist contradictory two demands, one for storing a sufficient amount of fluid so as to avoid fluid running out and another for storing a minimum amount of fluid so as to avoid causing resistance against the transmission mechanisms mounted inside of the vehicle frame. The prior frame structure is not designed to fully address these demands.
In consideration of the above prior art, it is an object of the present invention to provide a frame structure that ensures high efficiency assembly, while addressing the problems with the interconnected arrangement of a clutch housing, an intermediate housing and a transmission case along the vehicle longitudinal axis.
It is another object of the present invention to provide a frame structure that ensures high efficiency assembly as well as providing a free space above the intermediate housing, while addressing the problems with the interconnected arrangement of a clutch housing, an intermediate housing and a transmission case along the vehicle longitudinal axis.
It is still another object of the present invention to provide a frame structure that is capable of effectively preventing fluid for feeding to a HST unit running-out while keeping the amount of fluid stored in a reservoir tank as low as possible, which reservoir tank being constituted by at least a portion of an inner space of the frame structure that accommodates a running-power transmission path for transmitting power from an engine to drive wheels via the HDT unit and is arranged along the vehicle longitudinal axis extending from a first side to a second side of the vehicle.